The present invention relates to coherent optical processing systems in general, and in particular to such systems in which signals to be processed modulate a guided optical beam by means of an integrated optics assembly.
The introduction of an input signal to a coherent optical processing system is typically accomplished by means of an acoustooptical device. Examples of systems employing such devices, commonly known as Bragg cells, are described in U.S. Pat. No. 3,821,548, issued to Jernigan; and in "Multichannel Signal Processing Using Acoustooptic Techniques", by J. N. Lee et al, IEEE Journal of Quantum Electronics, Vol. QE 15, No. 11, November 1979, pp. 1210-1215.
Several drawbacks are inherent in the use of Bragg cells and similar acoustooptical techniques and are well known. Specifically, nonlinearities in the input signal-to-refractive index transfer function restrict the dynamic range of such devices. Other problems include lateral spreading of the acoustic beam in the direction of light propagation, the need for an electrode structure to launch the acoustic signal, frequency dependent attenuation of the propagating acoustic beam and the need for an acoustic absorber to prevent reflections from the waveguide edge.
U.S. Pat. No. 4,263,570, issued to DeFonzo et al is exemplary of various prior art microwave modulators employing area-modulated photoconductive elements. In the DeFonzo et al device, a change in the electrical conductivity of an inner wall in the microwave carrier waveguide alters the boundary conditions presented to the carrier, and as a result makes phase modulation possible for radiation (e.g., microwave, millimeter and submillimeter carriers) whose wavelength is comparable to the physical dimensions of the waveguide cavity. The inner wall of the waveguide is formed from a photoconductive material, and wave modulation in turn is achieved by illuminating the material in response to an input signal, in effect changing a wall boundary condition. The DeFonzo et al device can only form a uniform phase shift or modulation on the microwave beam, in reality a temporal phase change such as a time delay. The beam wavefront cannot be spatially modulated with a variable phase change across the wavefront. ln addition, the DeFonzo et al technique is not directly applicable to optical carriers, since wavelengths of infrared, visible, and ultraviolet light are orders of magnitude smaller than the microwave wavelengths.